The duration of the Q-T intervals (QTI) of the electrocardiogram and some indices of hemodynamic performance were evaluated during steady-state abnormalities of ionized calcium ([Ca++]) homeostasis in 16 anesthetized closed-chest dogs. [Ca++] was maintained approximately 60 per cent above or below normal. Progressive changes in QTI occurred for as long as 15 min after onset of steady-state alterations in [Ca++]. With institution of a hyper- or hypocalcemic plateau, the duration of the corrected intervals between the Q-wave deflection and the origin of the T-wave deflection (Q-oTc) and the Q wave and the end of the T-wave deflection (Q-Tc) were altered pari passu. In view of the progressive changes in QTI observed during the first 15 min of steady-state alterations in [Ca++], the magnitude of a disturbance in calcium ion balance must be evaluated by direct [Ca++] measurement. The data also indicate that alterations in [Ca++] are associated with changes in both ventricular and peripheral vascular function. Hypocalcemia was associated with a step increase in left ventricular filling pressure (LVFP) from 6.8 ± 0.6 to 9.7 ± 0.7 torr, mean · SEM, a step decrease in mean arterial blood pressure (MAP) from 108 ± 12 to 78 ± 15 torr, and a transient increase in cardiac output (CO) from 4.8 ± 0.2 to 5.7 ± 0.2 1/min, which was limited to the first 5 min of observation. With hypercalcemia, changes in CO and LVFP were insignificant, while MAP increased from 112 ± 6 to 123 ± 8 torr.